Convection and Critical Dynamics in Helium Three Superfluid Helium Four Mixtures

拟议的测量将使用超流氦混合物进行，将解决当前相当感兴趣的一些问题。使用一种利用热中子和氦3相互作用的开创性流动可视化技术，他们将获得液氦对流开始附近的第一张流动图像。其他测量将使用在低温下可获得的非常高的温度分辨率和本实验室开发的可变几何技术来进一步表征这些流动。超流体混合物有一个独特而有趣的参数范围，在这个参数范围内，定常对流的流体动力不稳定性会发生变化。使用适当的几何结构，他们将详细探索这些不稳定性，它们导致混乱的方式，以及它们受到稳定对流模式的影响的方式。在使用中子成像技术的第二组测量中，他们将建立氦的三个剖面，这三个剖面是由超流体混合物中的热流引起的。通过这些测量，他们希望了解最近的数据，这些数据表明，这些混合物中的热流明显违反了当前的理论。所涉及的理论--双流体流体力学和动态重整化群理论--的重要性，需要使用直接但非微扰的技术，如中子探测器，仔细研究混合物中的热流问题。

The proposed measurements, to be carried out using superfluid helium mixtures, will address a number of questions of considerable current interest. Using a pioneering flow visualization technique exploiting the interaction of thermal neutrons and helium three, they will obtain the first images of flows near the onset of convection in liquid helium. Additional measurements will use the very high temperature resolution obtainable at low temperatures and a variable geometry technique developed in this laboratory to further characterize these flows. Superfluid mixtures have a unique and very interesting parameter range in which the hydrodynamic instabilities of steady convection will change character. Using appropriate geometries, they will explore in detail these instabilities, the way they lead to chaos, and the way they are affected by the steady convection patterns. In a second set of measurements using the neutron imaging technique, they will establish the helium three profiles which are caused by the flow of heat in a superfluid mixture. By these measurements, they hope to understand recent data which show a striking violation of current theories describing the heat flow in these mixtures. The importance of the theories involved--Two-fluid Hydrodynamics and Dynamic Renormalization Group Theory--requires a careful examination of the heat flow problem in mixtures using a direct, but non- perturbative technique such as the neutron probe.